1. Field of the Invention
The invention relates to a data exchange system, comprising a fixed station and a portable data carrier which are coupled to one another by means for establishing a wireless connection for the transmission of high-frequency energy and data, the data carrier comprising at least an electric circuit with a supply device for generating the operating voltage for the circuit from the high-frequency energy received or from an energy source.
2. Discussion of the Related Art
A data exchange system of this kind is known from WO 92/08148. The data carrier of the system described therein serves to monitor a person in a security area. To this end, the data carrier periodically transmits very short, encoded information blocks which identity the data carrier and which are intercepted by an aerial and processed in a receiver. Moreover, at entrances of the security area there are provided interrogation devices which transmit a high-frequency electromagnetic field that also transfers information. When the data carrier approaches such an interrogation device, the energy taken up from the high-frequency electromagnetic field feeds the circuit of the data carrier and at the same time a battery accommodated in the data carrier is switched off, that is to say only for as long as the energy of the high-frequency field taken up by the data carrier is sufficiently high. The data carrier then returns its identification to the interrogation device which can thus grant access to the security area.
The interrogation device in this known system continually transmits a high-frequency field. Moreover, the interrogation device and the aerial with receiver constitute fully separate devices. Notably the tact that the interrogation device must continually transmit an electromagnetic field not only constitutes a kind of burden to the environment, but also gives rise to problems when the energy source present in the interrogation device is of limited capacity as is the case in, for example, a vehicle.
It is an object of the invention to provide a data exchange system of the kind set forth in which the power consumption in the fixed station is lower and which can be flexibly used for various applications.